It was my intention to have done this much more fully than in the present treatise; but having been interrupted by unforeseen circumstances, and being unwilling to delay any longer the publication of this essential part of my proposed plan, I have determined to submit it to the public in its present form; and will only add here a few observations on a question closely connected with it, which has lately excited much interest and discussion. Among the questions for the determination of which a standard measure of value is most particularly required, are those which relate to alterations in the value of the currency. We know perfectly well, from experience, that commodities are subject to great variations of price, and that many of these variations may arise from causes which alter the natural value of these commodities, and are equally applicable to a large mass of them, as to a very few. On the supposition of a large mass being altered, any article which had retained the same natural value, would have its power of purchasing considerably affected; but this would be owing to an alteration in the value of the mass of commodities, and not in the value of the article, which by the supposition remains the same. It follows, that although money may increase in its power of purchasing, it does not necessarily increase in value. But in estimating the value of money, some criterion or other must be referred to. If we cannot refer to the mass of commodities, we must refer to some one object, and this object can only be labour. Our present inquiry, therefore, must be into the causes which affect the value of the precious metals as compared with labour. These causes are of two kinds:--first, those which occasion a high or low rate of profits, which, as connected with the progressive cultivation of poorer land, and operating universally and necessarily on the precious metals in common with all other commodities, and raising or lowering them with regard to labour, may be denominated the primary and necessary cause of the high or low value of metallic money.--And secondly, those which depend on the fertility and vicinity of the mines; the different efficiency of labour in different countries; the abundance or scarcity of exportable commodities; and the state of the demand and supply of commodities and labour compared with money; which may be denominated the secondary and incidental causes of the high or low value of metallic money. These two different kinds of causes will sometimes act in conjunction, and sometimes in opposition, so that it may not always be easy to distinguish their separate effects; but as these effects have really a different origin, it is desirable to keep them as separate as we can.